Camera stops working close to ground

[mramato]

1. Go to http://localhost:8080/Apps/Sandcastle/index.html?src=Google%20Photorealistic%203D%20Tiles.html
2. Navigate to the soccer field right near the starting camera location:
   
3. Zoom in close.
4. You can no longer pan, the camera is mostly stuck. Here is my view when this happens:
   

This happens in a bunch of 3D Tiles tilesets, Google was just the easiest to explain.

I was hoping the recent picking fixed would have addressed this, but it's still an issue in main.

[cddhq]

I also encountered this problem where the custom material froze in a low perspective and stopped flowing, but the console did not report any error messages

[gammaxy1]

I found the code that causes the behavior, but it is protecting against issues with the camera panning below the ground so simply removing it is worse behavior.
https://github.com/CesiumGS/cesium/blame/5fda45a5861ca67291ff55655c9c395b8a06d20d/Source/Scene/ScreenSpaceCameraController.js#L1875

[ggetz]

Thanks for the clue @gammaxy1!